In the purification of river water, lake water, sea water, industrial waste water or the like, the turbidity material in water has heretofore been agglomerated with a flocculant such as polyaluminum chloride or the like to form flocks. A method of purifying water by adding a flocculant (flocculation method) is described in detail in, for example, "Josui no Gijutsu" (Technique of Water Purification) by Kenji TANBO and Koichi OGASAWARA published by Gihodo (1985), Chapter 2. However, in the case of such a method, when the quality of raw water such as river water, ground water, waste water or the like is fluctuated, it is necessary to optimize the amount of a flocculant added, pH and the like corresponding to the fluctuation in order to carry out an effective treatment, and a preliminary test such as a so-called jar test is required. In the conventional water-purification method, not only is the operation thus complicated, but also large scale equipment such as a flocks-forming pond, a settling pond, sand-filtration equipment or the like has been required.
In order to overcome the above-mentioned defects which the flocculation method has and to obtain the stabilized water quality by compact equipment without being greatly affected by the fluctuation of the quality of the raw water, a new process for treating water using a membrane has been proposed.
It is difficult to employ membranes alone in industrial uses. However, from the viewpoint that the use of a membrane enables one to ensure a high treatment capacity per unit volume at a low cost, a hollow fiber membrane made of a polymer has been put to practical use as a membrane module. The hollow fiber membrane module can be constructed by receiving such a hollow fiber membrane into a module case, bonding and fixing at least one end of the membrane with a thermo-setting resin such as an epoxy resin, a urethane resin or the like, thereafter cutting the unnecessary bonding portion, and opening the hollow portion of the fiber.
However, when the above-mentioned membrane module is used, there has been such a problem that the permeation flow velocity of the treating water is decreased with the lapse of time (New Membrane Technology Symposium '92 held by the Membrane Society of Japan and the Japan Management Association, Preprint SESSION 3, 1992).
For solving these problems, there has recently been proposed a method of inhibiting the decrease of permeation flow velocity of the treating water with the lapse of time by previously treating the raw water with ozone [Jour. American Water Works Association (AWWA), 77 (60-65) (1985)]; a method of inhibiting the clogging of a filtration membrane by injecting ozone into the raw water before membrane filtration and then filtering the raw water containing ozone (JP-A-4-108518); an oil-water separator in which ozone or hydrogen peroxide is fed before membrane filtration in separating an oil and water from waste water containing the oil (JP-A-63-93310); a method of recovering the decreased permeation flow velocity by use of water containing ozone when a separating membrane is washed (JP-A-3-249927); or the like.
Such an ozone treatment is particularly effective when the clogging of a membrane results from an organic material; however, a membrane module capable of resisting the strong oxidative action of ozone is required. For example, when water containing ozone is filtered using an organic hollow fiber membrane, the membrane constituting the membrane module, the module case portion and the bonding portion must have not only the respective functions as the respective constructive materials, but also ozone resistance in at least portions contacting an ozone gas or water containing ozone. In addition, from the industrial viewpoint of the life of the membrane module, ozone resistance of at least one-half year, preferably 1 to 3 years is required.
As a membrane module having ozone resistance, there have heretofore been known, for example, inorganic membrane modules using a ceramic membrane, ALL-PVDF Ultrastack.TM. using the flat membrane-like ultrafiltration membrane made of a polyvinylidene fluoride (PVDF) stated in ULTRAPURE WATER, 7 (6), 32-36, 38-40 (1990), and the like. However, the module using a ceramic membrane is expensive and has such a problem that there is a limit in the compactification of equipment. Also, in the membrane-filtration system using ALL-PVDF Ultrastack.TM., the compactification of equipment is difficult and, in addition, when it is intended to adopt a cross flow filtration system which is excellent in respect of stably maintaining the permeation flow velocity, there is a problem that said application is difficult.
Furthermore, when the raw water contains inorganic materials such as sand and clay, even if an ozone treatment is effected, there are such problems that inorganic materials accumulate on the membrane surface and the permeation flow velocity is decreased with the lapse of time. For solving these problems, JP-A-7-265671 discloses an external pressure cross flow filtration which can make it difficult for materials causing clogging to pile up on the membrane surface and can make the membrane surface area large, and an external pressure system membrane module capable of discharging the suspended materials which have piled up between the hollow fiber membranes by routine aeration-flushing (stated as an air-bubbling). However, these membrane modules use a silicone rubber as an adhesive for bonding and fixing the membrane used on the module case, and hence, they lack strength and are inconvenient in conducting a stable filtration on a commercial scale over a long period of time though they can be applied to a short time use or to a use in a small module diameter. Therefore, a development of a more improved membrane module has been strongly desired.